Make parsetree expression attributes immutable

[cknitt]

Summary

Make pexp_attributes immutable in the current parsetree. The editor's “Add braces to function” action was its only in-place writer; the Pexp_braces change in #8678 now constructs a new expression instead.

The frozen parsetree0.ml stays unchanged for external PPX compatibility. This PR is stacked on #8678 and continues #8624.

Verification

• make test
• make test-analysis

[pkg-pr-new]

Open in StackBlitz

rescript

npm i https://pkg.pr.new/rescript@8685

@rescript/belt

npm i https://pkg.pr.new/@rescript/belt@8685

@rescript/darwin-arm64

npm i https://pkg.pr.new/@rescript/darwin-arm64@8685

@rescript/darwin-x64

npm i https://pkg.pr.new/@rescript/darwin-x64@8685

@rescript/linux-arm64

npm i https://pkg.pr.new/@rescript/linux-arm64@8685

@rescript/linux-x64

npm i https://pkg.pr.new/@rescript/linux-x64@8685

@rescript/runtime

npm i https://pkg.pr.new/@rescript/runtime@8685

@rescript/win32-x64

npm i https://pkg.pr.new/@rescript/win32-x64@8685

commit: cfd2c2a